Throughout this specification the use of the word “inventor” in singular form may be taken as reference to one (singular) inventor or more than one (plural) inventor of the present invention. The discussion throughout this specification comes about due to the realisation of the inventor(s) and/or the identification of certain prior art problems by the inventors.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.
It has been realised that in applications where RFID and remote powering is used and where orientation of the items to be identified cannot be guaranteed, such as shelving and storage systems, document tracking, luggage identification, gaming tokens, jewellery and diamond identification by way of example only, items to be identified may be missed and/or not correctly identified.
The applicants are aware of a number of transponder systems that provide one dimensional, two dimensional, limited three dimensional or full three dimensional capability. These systems utilise a multiplicity of interrogator coils operating in different coordinate axis, to achieve two or three dimensional operation.
One particularly advantageous interrogator design produces a uniform field in three dimensions. This form of interrogator is known as a Tunnel Reader Programmer (TRP). An example of a TRP for interrogating transponders on pallets or conveyors which meets all OH&S and EM regulations in Australia is disclosed in U.S. Pat. No. 5,258,766 and international application PCT/AU95/00436.
While a TRP has excellent three dimensional interrogation properties, a major drawback is that it is only suitable for applications where the RFID transponders are moved in and out of the TRP, usually on a conveyor or similar. TRP are inherently unsuitable for applications requiring the interrogation to occur on a flat surface such as a table or wall. For these applications flat planar antenna coils are required however these coils suffer from producing fields in only one direction at any point relative to the coil and do not have a two or three dimensional interrogation capability.
FIG. 1 illustrates a conventional planar antenna coil arrangement, in which the coil 10 has windings 11 arranged in a somewhat circular configuration.
FIG. 2 illustrates a cross sectional view X of FIG. 1 of the windings of the coil of FIG. 1. The magnetic field created by inducing power into the windings is represented 12. If a transponder 13 has a coil (not shown), but placed on it's outer top surface, for example, and if the transponder 13 is positioned substantially horizontally between the windings as illustrated in FIG. 2, the field 12 produced by the windings 11 has a correct orientation to power the transponder. Equally, if a transponder 14 is placed in a substantially vertical orientation as illustrated in FIG. 2, it too will be powered by the field 12. However, if a transponder 15 is placed substantially horizontally near or outside the windings 11, the field 12 generated by the windings will not be correctly oriented to power the transponder 15. Likewise if the transponder is placed in a substantially vertical orientation in the inside of the windings 11 and 12 as illustrated in 16 the field 12 generated by the windings will not be correctly oriented to power the transponder 15.
A flat planar arrangement of antenna coils which can provide two dimensional or three dimensional interrogation is shown in international application WO 2007/030861A1. WO 2007/030861A1 is incorporated here in by reference.
A one dimensional field can be generated over an extended area by overlapping planar coils that are switched in a sequential fashion. WO2007/030861A1 shows planar coils arranged in an overlapping fashion that are switched in a sequential fashion in order to generate a two dimensional interrogation field. WO 2007/030861A1 further shows that arranging a second layer of planar coils that are orthogonal to the first layer generates a three dimensional interrogation field. The coil antenna arrays for both the two dimensional and three dimensional operation are sequentially operated in order to provide their respective two and three dimensional interrogation fields.
FIG. 3(a) illustrates the prior art coil arrangement 333 and 334 of WO 2007/030861 A1 where the coil windings 331 and 332 from several coils have been overlapped in order to form a series of parallel spaced conductors through which currents are sequentially switched in order to produce both tangential and normal magnetic field components. The spatial relationship of the sequentially switched currents is chosen to ensure that at different times a tangential and a normal magnetic field are produced at the same location. The conductors are preferably arranged in a planar fashion and the tangential and normal magnetic fields are produced above the planar surface. A single layer of parallel spaced conductors provides for two dimensional operations.
FIG. 3(b) illustrates the prior art coil arrangement 333 of WO 2007/030861A1 where adding a second parallel layer 334 of orthogonally oriented parallel spaced conductors 332 provides three dimensional operations where currents are sequentially switched in both layers 331 and 332.
The amount of overlapping between the planar coils and the orthogonal coils can be adjusted to minimise the mutual coupling between the coils. This is advantageous as it reduces parasitic interactions between the coils. WO 89/10030 shows this method of minimising the mutual coupling between antenna coils which is advantageous for producing large arrays of many antennas.
Producing a two or three dimensional interrogation field and minimising the mutual coupling may be contradictory requirements and other methods of reducing the effective coupling between coils may be required. The parasitic coupling between antenna can be reduced by using a switched device, or multiple switched devices, to open inactive antenna coils. Switch devices can be relays, MEMs or PIN diodes or any other device capable of interrupting an RF signal. PIN diodes are circuit elements specifically designed for the purpose of providing a controllable RF switch. The method of using PIN diodes to open circuit RF signals is described for example in WO2007/030861A1, WO2005/083893, WO2005/062421 and WO2000/067395. Opening the circuit of inactive antenna coils reduces the parasitic coupling between coils as little or no current can flow through the open switch(s) in the inactive coil(s).
In order to sequentially operate the antenna arrays described in WO 2007/030861A1 the interrogating signal is sequentially switched to each antenna coil in the antenna array. An interrogator or reader with multiple outputs where the interrogation signal can be sequentially switched between the outputs is particularly advantageous for operating the antenna arrays described in WO 2007/030861A1.
An example of such a reader is shown in U.S. Pat. No. 6,903,656 which shows a reader where an antenna switch is located at the output of the reader. The antenna switch is integral to the reader and is directly controlled by the reader's digital controller. Multiple antennas are operatively connected to the antenna switch by connecting cables. U.S. Pat. No. 6,903,656 is directed towards antenna tuning methods and does not deal with the application of the switched interrogation signal for generating two or three dimensional interrogation fields.
There are applications where it is advantageous to move the antenna switch out of the reader and locate it remotely between the reader and the antennas connected to the switch. When positioned remotely the RF switch is called an RF multiplexer or RF MUX. The cable connections between the RF MUX and the antennas are considerably shorter. Remote positioning can save considerable cable length in installation since only a single cable connects the reader to the RF MUX replacing the multiple cables from the reader to each of the antenna that would have been required had the antenna switch been located in the reader.
WO2007/094787A1 shows an RF MUX where the RF MUX is located remotely from the reader and is connected to the reader by a single antenna cable. The single antenna cable is used to convey the interrogation signal, DC power to power the MUX, modulations of the interrogation signal to control the MUX's operation and information from the MUX sent back to the reader by RF backscatter of the interrogation signal. The output ports of the MUX are connected to antennas or other MUXs by further cable. The method of using the RF interrogation signal for controlling the MUX and sending information as a backscatter signal back from the MUX to the reader requires complex RF circuits that can couple to, demodulate and backscatter the interrogation signal. A serious disadvantage of the method of data signalling described in WO2007/094787A1 is the circuit complexity and cost.
Another problem exists whereby a person installing an RFID system and an antenna is coupled to a reader. However, the each antenna type has certain characteristics, and also each reader is usually configured to operate with a certain type of antenna. If an antenna is coupled to a reader, the reader should be configured to operate to that antenna type. However, this configuration is usually done manually, if it is done at all, and often, the configuration is not done correctly.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.